function mymodule_init(){
  //get variable content (last cron run time), if not exists, return 0
  $last_cron_run = variable_get('mymodule_cron_run', 0);

  //if it was more than 5min before
  $five_minutes = 5 * 60;
  if($last_cron_run + $five_minutes < time()){
    //run cron
    $result = drupal_cron_run();

    //do with $result what needed

    //set last cron run time to variable
    variable_set('mymodule_cron_run', time());
  }
}